Field of the Disclosure
This application generally relates to circuits and packaging for fingerprint sensors.
Background of the Disclosure
Capacitive sensing of fingerprints provides for collection of fingerprint information in response to distinct measures of capacitance between, on the one hand, one or more capacitive plates in a fingerprint recognition sensor, and on the other hand, ridges and valleys of a user's finger (such as the epidermis of the user's finger, or possibly, subdermal layers of the user's finger).
It sometimes occurs that measurements of capacitance involve introduction of electric charge on the epidermis of the user's finger. This can have the effect that only a small amount of charge can be introduced without the user feeling the charge, sometimes as a tingling or other noticeable effect on the epidermis of the user's finger.
It sometimes occurs that measurements of capacitance involve relatively small differences in capacitance between, on the one hand, the capacitive plates of the fingerprint recognition sensor, and on the other hand, the ridges and valleys of the user's finger. For example, this would involve placing the user's finger as close as possible to the capacitive plates. This can have the effect of limiting the design flexibility for the fingerprint recognition sensor.
It sometimes occurs that measurements of capacitance involve positioning of the user's finger with respect to the fingerprint recognition sensor. For example, the user's finger might have to be placed within a conductive ring, significantly limiting the size and position of the fingerprint recognition sensor. This can also have the effect of limiting the design flexibility for the fingerprint recognition sensor.
It sometimes occurs that measurements of capacitance involve capacitive coupling with a portion of the user's finger other than the epidermis. For example, capacitive coupling (or other fingerprint recognition sensing) might involve as a subdermal layer of the user's finger. This might involve introduction of a relatively greater electric charge to conduct that capacitive coupling. As described in part above, this can have the effect that the user might feel the charge, sometimes as a tingling or other noticeable effect in a portion of the user's finger.
Each of these examples, as well as other possible considerations, can cause difficulty for the fingerprint recognition sensor, and for the device incorporating the fingerprint recognition sensor (such as a computing device using fingerprint recognition for authentication). The fingerprint recognition sensor might be limited in size, or position, or in whether it can be relatively easily incorporated with other elements of the device incorporating the fingerprint recognition sensor. For a first example, this can have the effect that the fingerprint recognition sensor might not be easily incorporated into some types of devices (such as relatively small devices like smartphones and touchpads). For a second example, this can have the effect that the fingerprint recognition sensor might be required to be relatively fragile or otherwise subject to unwanted design constraints.